Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printer. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make a final color image. For example, in the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptor is exposed to a light image which represents a first color image, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The charge, expose, and develop process is then repeated for a second color (say yellow), and then possibly for a third color (say magenta) and a fourth color (say cyan). If the various colors of toner particles are suitably registered a desired composite color image results. However, if the toner layers are not placed in a superimposed registration, but are rather placed in discrete areas, a highlight image results. Highlight color machines typically use at least black and an additive toner such as red. For example, most of a bill might be printed in black, but the payment due might be printed in red so as to stand out from the rest of the bill. In any event, the final image is then transferred and fused onto a substrate.
The REaD IOI electrophotographic printing process described above is usually performed in a machine in which "subtractive" color toners, usually cyan, magenta, and yellow, are overlaid to result in a final image having various colors and color tones. Subtractive colors absorb one third of the visible spectrum. While this process, referred to hereinafter as image-on-image color printing is very useful when producing image pictorials, image-on-image color printing is not optimum for highlight color printing. In highlight color printing a highly saturated color, such as deep red, is usually desired. Unfortunately, image-on-image color printing is inefficient when producing colors such as deep red because subtractive toners are usually not able to produce the desired color hue and chroma.
Red, green, and blue are "additive" colors. Additive colors absorb two thirds of the visible spectrum. For this reason they are unsuitable for image-on-image color printing because placing two colors of toner on top of each other results in black (or dark brown). Thus additive color toners cannot be used in image-on-image color REaD IOI printing. Thus with additive color toners it is very important to suppress, rather than to enhance image-on-image (IOI) development.
U.S. Pat. No. 5,828,933, which issued on OCT. 27, 1998 entitled "ADDITIVE COLOR RECHARGE, EXPOSE, AND DEVELOP ELECTROPHOTOGRAPHIC PRINTING" teaches how to perform additive color REaD printing. That patent discloses three basic ways of inhibiting image-on-image printing. First, by setting the exposure set point (threshold) such that exposure losses when imaging through an existing toner layer reduces the electrostatic developing potential such that toner is only deposited on "bare" (no toner) areas of a photoreceptor. Second, by using a DC corona system (such as a DC scorotron) for recharging so as to enhance the voltage drop across previously developed toner. Third, by increasing the developed toner mass so as to increase the dielectric properties of developed toner. However, this process is somewhat complicated and difficult to implement.
Therefore, an alternative technique of achieving inhibited image-on-image REaD printing when using additive toners would be beneficial.